Site Search Navigation

Site Navigation

Site Mobile Navigation

Searching for the Dark Side

By Chris Lintott March 11, 2007 9:16 pmMarch 11, 2007 9:16 pm

Spiral Galaxy ESO 269-57. (European Southern Observatory)

It is possible to write the history of our increasing knowledge about the universe and our place in it as a series of blows to human pride. We discovered that the Earth and then the Sun are not the center of the universe, that we do not reside at the center of the galaxy and that there is nothing particularly special about our own Milky Way system. The latest such shock is that the kind of matter we – and everything we can see directly – are made of accounts for only a sixth of the total mass in the universe. In the absence of knowledge, we label the rest as “dark matter” and look to detect it via its effects on what we can see.

We have long known that there is more to our universe than we can see. The great eccentric of American astronomy, Fritz Zwicky, realised in 1930 that the galaxies within galaxy clusters were moving more rapidly than they should be. The only explanation was to assume that there was more matter present than scientists had thought, so that the increased gravitational attraction could prevent the galaxies from escaping. A similar problem exists on the scale of individual galaxies; if only the visible disk of a galaxy such as the Milky Way existed, the galaxy would fly apart in just a few million years. The solution is the same; if the Milky Way is embedded in a halo of dark matter, then all is well.

What is this dark matter? We know very little about it; whatever it is, it interacts via its gravitational attraction with normal matter, but not with light. For a while the best candidates were objects such as black holes, or free-floating planets in the Milky Way halo, but searches for the effect that such objects would be expected to have on background stars showed that very few exist. Instead, most cosmologists now believe that dark matter is composed of slow-moving exotic particles that have yet to be identified.

Such a situation is unsatisfactory to say the least. Although particle physicists are working on the problem, and experiments such as the Large Hadron Collider, a subatomic-particle accelerator being built at CERN in Geneva may identify dark matter candidates, it is embarrassing not to know what the main component of the universe is. The alternative is to assume that there is something wrong with our knowledge of gravity, and that the work of Einstein needs revising.

The matter of the Bullet Cluster. (NASA)

Theories that attempt to do just that have been somewhat successful. The current battleground is known, appropriately enough, as the Bullet Cluster of galaxies. The picture above needs a little explanation; the pink is emission from hot gas within the cluster, seen in X-rays. The blue region represents the underlying mass distribution, as determined by studying the distortion in the shape of the background galaxies as light passes through the Bullet Cluster in the foreground.

The Bullet Cluster is actually two clusters of galaxies in collision. As they hit, the gas contained within each interacts and heats up, leading to the X-ray emission we observe, and the gas slowing down. The dark matter, however, interacts only via gravity – a much weaker force – and so overshoots the center. This cluster is important because it allows us to actually see the difference in behavior between ordinary and dark matter.

These observations are a stringent test for theorists attempting to dispose of dark matter, but I for one hope they succeed. It would be tidier, somehow, to lose the enigmatic dark matter, and exciting to discover a successor to Einstein’s relativity. As George Bernard Shaw said in 1930, “Ptolomy invented a universe and it lasted 2000 years. Newton invented a universe and it lasted 200 years. Now Dr. Einstein has invented a new universe, and no one knows how long this one is going to last.”

A terrific way of keep up with the advances in astronomy and physics that are coming in so fast now. And to update old textbooks on astronomy.
I am using these three articles and other info from the Times to augment the education of young, bright Belizean kids in San Pedro on the island of Ambergris Caye, Belize.

I believe dark matter may be a quality of space having mass and other attributes, not just an emptiness, a void. This mass quality of space would allow it to be a medium for light wave and energy transmission. Wave energy such as sound needs a medium to generate, so how could light waves generate in an absolute vacuum? Ancient scientists recognized space is a substance, not an absence of substance, and identified it as one of the four elements, ether. It may be this mass quality of space, not the speed of distant galaxies, that causes the observed red shift in light from distant objects in the universe. Distant galaxies therefore would not be as far away or receding at the very high speeds that astronomers assume is necessary to produce the red shift.

Good article. It would be interesting to live 5,000 years from now when humans may be exploring our own Milky Way galaxy. Look back 5,000 years and where were humans then? Throwing rocks at one another, I think. Now we merely threaten nuclear devastation. But a rocket fuled by nuclear power is not too far away. And when one is developed, we should be able to send astronauts to Venus and Mars.

These are wonderful discussions and add so much to the Times. I agree entirely with the earlier comment. I was also amazed to read of Brian May’s background!

Somehow, I’m afraid that the scientific endeavor is likely to be far more humbling in the end (whenever that may be). The endless proliferation of subatomic particles and ever more exotic theories seems to me to be nothing more than a somewhat futile effort to conceptualize phenomenon in mathematical terms and ordinary constructs that are literally beyond our ability to know. I wonder if we are not in the dilemma of the inhabitants of Flatland, trying to imagine the reality of three-dimensional space when our experience does not allow us to know anything beyond two-dimensions. Multi-dimensional universes are the result of mathematical models; but what those dimensions actually are, and how they would explain phenomenon in our three or four-dimensional space is impossible to know. Some of the most sophisticated observational theories rest upon the simplest of assumptions. Although there is no “better” model, I sometimes wonder what would happen to our notion of the universe if we learned that the relationship between velocity (red-shift) and distance to an object were wrong…

Jim Auster starts his ruminations with “I believe …” Good thing, too, because I doubt he has much more than belief to back him up. That’s not to say he’s wrong, however. Perhaps, someday, we will learn that dark matter does function, after all, very much like the ether of the ancients. I would not be quite as ready as our friend to begin postulating off his belief. The danger is that one becomes tempted to use the postulations as evidence that the initial proposition must be true. That commits the logical fallacy of the reverse conditional (if A then B, B is true, therefore A is true).

We are definitely on the cusp of a radical reappraisal of our ideas about the universe, even what is our place in it. More advanced ideas even than dark matter and dark energy were covered in that excellent article in the Sunday Magazine. The wilder mathematics of string theory has us swimming in a sea of infinite universes, all based on logical constructs that are undetectable to us. Some of the newest theories of dark matter say there isn’t any. Instead, we are feeling the effects of overlapping other dimensions on our own. Even The Great Barrier—what existed before space and time came into existence—can be breached, say some theorists.

My favorite quote from Emerson: as true today as when he wrote it in 1844.

“We think our civilization near its meridian,
but we are yet only at the cock crowing and the morning star.”

“This world is not conclusion — a species stands beyond, invisible, as music, but positive, as sound. It beckons, and it baffles — philosophy don’t know — and through a riddle, at the last, sagacity must go. To guess it, puzzles scholars, to gain it, men have borne contempt of generations and crucifixion, shown.”

There are simple alternatives to the new physics, but it’s hard to see what they are since there is a real problem getting something published if it does not conform to the standard model. Also, the 11 dimensional strings just don’t cut it with Occam’s razor. I think the universe is much simpler than we seem to want to make it to be.

…expanding on my earlier ruminations and postulations, if space itself has or is the mass and gravity attributed to “dark matter”, the observed expansion of the universe may be caused, not by (postulated)repulsive forces of “dark energy”, but by the gravitational pull of endless space beyond the galaxies pulling the galaxies outward….

Certainly provides ‘food for thought”. What can be seen can I believe, eventually, be understood. What an exciting time we live in. To stare into the void, much as the cave man and women did, and wonder what really lays out there in actual composition in the bright mysterious firmaments!

I sometimes wonder when this current marvelous serendipitous balance of the conditions on planet Earth in this solar system that have led to the formation and preservation of life as we know it will end? Will the earliest interaction of our sun’s radiation creating our current atmosphere by its effect on the original gases in Earth’s early atmosphere (ammonia, methane and water vapor) and then forming the 4mm ozone layer that led to the protection and maintenance of this current atmosphere have occurred for naught? Will we humans be stupid enough to continue to increase the CO2 level in our atmosphere so rapidly that the heat produced will end life here in a few hundred years instead of the 5+ billion years it took to create it? Or will we destroy the ozone layer by allowing the inevitable escape of leaking hydrogen gas to escape into the stratosphere from our attempt at eliminating fossil fuel CO2 pollution and kill life much quicker in that way?
At any rate, the sun will eventually burn out anyway and end life as we know it, but that will be several billion years from now. Wouldn’t it be better to keep this planet balanced as it currently is for as long as we can to be able to evolve enough to learn how to travel to another system to at least continue evolving. We only utilize 10% of our brain at this point; think how much we might be able to do if we used 100% of it! This blog has shown all how beautiful the universe is; let’s wake up and do what’s necessary to preserve it as long as we can!

Maybe astrophysists just has to recalculate total matter in the universe. Maybe current theoretical calculations are off by a 1/6th. Maybe the string theory can account for this lost matter….just about anything is possible under the string theory isn’t it.

Back in the 1930’s physicists believed that based on the Bohr Atom they had a very complete understanding of just about everything there was to know about nuclear physics and astronomy, except for a few, shall we say, niggling details. The latter turned out to be the tips of a vast field of icebergs hiding the mind-boggling complexity of sub-atomic physics discoveries that followed. Dark matter/energy are a mere decade old but once again have threatened to upset the old order. And so it is, not that the world is really simpler than we’d like it to be, but uncountable orders of magnitude more complex. In thinking that we can ever nail it down completely with our puny mathematics and physics and intellects of today is just man’s irrepressible arrogance of the moment oblivious to its own unfathomable ignorance. We are still children playing in the garden — and so will remain. Those astoundingly brilliant Physicists have learned ot enjoy the mystery as an endless voyage of discovery of the wonders of creation — and so should we. As my teenage daughter used to say, “Deal with it, Dad!”

Nothing wrong with denting human pride. We should welcome it. A new scientific context; theories of gravity beyond Einstein; what’s not to welcome ? The Kantian notion that we shall never know ultimate truths should not take away the adventure of trying, and advancing, millimeter by millimeter, sometimes less, sometimes more.

Excellent posting! I’d like to add that there are more roads to identifying dark matter than production at the LHC, which is often given the most press (because it costs the most money). Several experimental collaborations around the world (e.g. CDMS and XENON in the U.S.) are running sensitive detectors in the hopes of directly detecting the rare impacts of dark matter particles on atoms here on earth. Other groups look for signals of dark matter annihilation in regions of high density, such as the centers of galaxies. It’s entirely possible for one of these experiments to see a signal while the LHC does not. The most exciting possibility, of course, is to combine data from multiple experiments to learn more about dark matter than any single experiment could.

The concept of time in Relativity as an absolute dimension continuous from the past to the future is fundamentally different from that of a variable with the uncertainties of Quantum Mechanics. This conflict between the two major theories of modern physics indicates a need to modify our concept of time.

To answer Sverre (#17) which I believe is in part a response to mine is that many of the advances we have made in physics are actually simplifying, not making things more complicated. Relativity is actually very simplifying, showing that symmetry is the overriding factor and that the world is quite illogical without it. The same with Newton, Copernicus et. al. Quantum mechanics doesn’t seem to follow that trend, but it appears we don’t really understand it so we make it complicated. If we add more complexity, we can always describe something better. Give me enough variables and I’ll draw you an elephant, and one more and I’ll make it wag it’s trunk.

To talk about how we are so puny and could never understand the world is profoundly depressing and does not really track with reality. We can do and understand amazing things, as long as we don’t let the limits imposed by anything other than the laws of nature get in the way.

“The alternative is to assume that there is something wrong with our knowledge of gravity, and that the work of Einstein needs revising.”

What is wrong with our understanding is the persistant belief with no evidence to support it, that gravity is a force. Gravity is not a force, it is an effect. It is an effect of the inherent curvature of space. Once we stop imagining that space is inherently Euclidean and has to be “bent” into curvature by the “force” of gravity, it is a short step to realizing that space is inherently curved, and there is no outside force bending it into curvature.

Since Prof Einstein already described just how curved space is – enough time and enough magnifying power, the person peering through that all powerful telescope will see the back of their own head – there is no need to revise anything, except the order in which we conceive of things. Mass is merely more deeply curved space.

Of course, this means giving up the modern era’s version of a flat earth: the big bang is really a big convergence, a continous fountain of creation, spewing forth ‘new’ matter and energy as the ‘old’ converges upon it continuously. That is how deeply curved space is: it makes it appear that the motion of galaxies is accelerating in a centerless expansion. Perpetual recycling is a better description. That background radiation, 2.7K or thereabouts? Part of the eternal return, along with the effect of gravity.

#7 – I think I agree with the spirit of your comments, but I do take umbrage at the word ‘futile’. It depends what your goal is, you see. I’m prepared to concede that we might never get to a final theory, but the progress we’re making – as we’ve tried to show in the columns published so far – is rewarding in itself. Isn’t it?

#21 – I agree entirely with your point about other experiments, and I’m sorry lack of space (and training!) precludes me giving a proper overview. I should add, though, that even if we find a dark matter candidate with any collider, we still need to confirm that those particles really are what we see the effect of up in space.

In general, I’m always interested in the fact that many of the discussions that lead from topics like this bring up string theory, despite the fact I didn’t mention it. Why does it have such a hold in the public consciousness?

What's Next

About

Sir Patrick Moore has been the host of “The Sky at Night,” a monthly television show about astronomy on the BBC, for 50 years. He is the author of more than 60 books on astronomy, and his own studies have focused on the Moon. Moore is also an accomplished xylophone player and composer. He is a coauthor, with Chris Lintott and Brian May, of “Bang!: The Complete History of the Universe,” which will be published in the United States this fall.

Brian May, best known as a guitarist, songwriter and performer for Queen, began his doctoral studies on the subject of interplanetary dust before the band hit it big in the early 1970s. Through the years, he retained a strong interest in astronomy, appearing regularly on Moore’s TV show, “The Sky at Night.” May has recently returned to his studies in astrophysics. He is a coauthor of “Bang!”

Chris Lintott, has been a co-host (with Sir Patrick Moore) of “The Sky at Night” since 2000. He recently completed his doctorate and is now studying star formation at the University of Oxford. He is a coauthor of “Bang!”